Re: [PATCH] dax: fix deadlock in __dax_fault

[Ross Zwisler]

On Thu, Sep 24, 2015 at 12:52:25PM +1000, Dave Chinner wrote:
> On Wed, Sep 23, 2015 at 02:40:00PM -0600, Ross Zwisler wrote:
> > Fix the deadlock exposed by xfstests generic/075.  Here is the sequence
> > that was causing us to deadlock:
> > 
> > 1) enter __dax_fault()
> > 2) page = find_get_page() gives us a page, so skip
> > 	i_mmap_lock_write(mapping)
> > 3) if (!buffer_mapped(&bh) && !buffer_unwritten(&bh) && !vmf->cow_page)
> > 	passes, enter this block
> > 4) if (vmf->flags & FAULT_FLAG_WRITE) fails, so do the else case and
> > 	i_mmap_unlock_write(mapping);
> > 	return dax_load_hole(mapping, page, vmf);
> > 
> > This causes us to up_write() a semaphore that we weren't holding.
> > 
> > The up_write() on a semaphore we didn't down_write() happens twice in
> > a row, and then the next time we try and i_mmap_lock_write(), we hang.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > Reported-by: Dave Chinner <david@xxxxxxxxxxxxx>
> > ---
> >  fs/dax.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 7ae6df7..df1b0ac 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -405,7 +405,8 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> >  			if (error)
> >  				goto unlock;
> >  		} else {
> > -			i_mmap_unlock_write(mapping);
> > +			if (!page)
> > +				i_mmap_unlock_write(mapping);
> >  			return dax_load_hole(mapping, page, vmf);
> >  		}
> >  	}
> 
> I can't review this properly because I can't work out how this
> locking is supposed to work.  Captain, we have a Charlie Foxtrot
> situation here:
> 
> 	page = find_get_page(mapping, vmf->pgoff)
> 	if (page) {
> 		....
> 	} else {
> 		i_mmap_lock_write(mapping);
> 	}
> 
> So if there's no page in the page cache, we lock the i_mmap_lock.
> The we have the case the above patch fixes. Then later:
> 
> 	if (vmf->cow_page) {
> 		.....
> 		if (!page) {
> 			/* can fall through */
> 		}
> 		return VM_FAULT_LOCKED;
> 	}
> 
> Which means __dax_fault() can also return here with the
> i_mmap_lock_write() held. There's no documentation to indicate why
> this is valid, and only by looking about 4 function calls higher up
> the stack can I see that there's some attempt to handle this
> *specific return condition* (in do_cow_fault()). That also is
> lacking in documentation explaining the circumstances where we might
> have the i_mmap_lock_write() held and have to release it. (Not to
> mention the beautiful copy-n-waste of the unlock code, either.)
> 
> The above code in __dax_fault() is then followed by this gem:
> 
> 	/* Check we didn't race with a read fault installing a new page */
>         if (!page && major)
>                 page = find_lock_page(mapping, vmf->pgoff);
> 
> 	if (page) {
> 		/* mapping invalidation .... */
> 	}
> 	.....
> 
> 	if (!page)
> 		i_mmap_unlock_write(mapping);
> 
> Which means that if we had a race with another read fault, we'll
> remove the page from the page cache, insert the new direct mapped
> pfn into the mapping, and *then fail to unlock the i_mmap lock*.
> 
> Is this supposed to work this way? Or is it another bug?
> 
> Another difficult question this change of locking raised that I
> can't answer: is it valid to call into the filesystem via getblock()
> or complete_unwritten() while holding the i_mmap_rwsem? This puts
> filesystem transactions and locks inside the scope of i_mmap_rwsem,
> which may have impact on the fact that we already have an inode lock
> order dependency w.r.t. i_mmap_rwsem through truncate (and probably
> other paths, too).
> 
> So, please document the locking model, explain the corner cases and
> the intricacies like why *unbalanced, return value conditional
> locking* is necessary, and update the charts of lock order
> dependencies in places like mm/filemap.c, and then we might have
> some idea of how much of a train-wreck this actually is....

Yep, I saw these too, but didn't yet know how to deal with them.  We have at
similar issues with __dax_pmd_fault():

	i_mmap_lock_write(mapping);
	length = get_block(inode, block, &bh, write);
	if (length)
		return VM_FAULT_SIGBUS;

Whoops, we just took the mmap semaphore, and then we have a return that
doesn't release it.  A quick test confirms that this will deadlock the next
fault that tries to take the mmap semaphore.

I agree that we need to give the fault handling code some attention when it
comes to locking, and that we need to have better documentation.  I'll work on
this when I get some time.

In the meantime I still think it's worthwhile to take the short term fix for
the obvious generic/075 deadlock, yea?

- Ross
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